The use of machine directional oriented (MDO) films made from polyethylene is well known. These films are generally produced to down gauge existing blown film recipes. This means less polymer film is required to achieve a target end use.
Linear low density polyethylenes (LLDPEs) such as Borlite OPE grades from Borealis enable down gauging as they give extremely high mechanical strength after the MDO step. However, even these film grades can still be improved upon. A problem with MDO film is a tendency to split in the machine direction. We have all observed a package splitting and watched that split propagate down the package, leaving the contents of the package spilled.
The present inventors therefore sought a solution to the problem of tear without compromising other properties of the film, in particular the impact strength of the film. The inventors have found that by using a certain multimodal LLDPEs, tear can be markedly improved without significantly compromising the mechanical properties of the film, e.g. in terms of impact strength. The increase in tear is only observable however in MDO films of a particular minimum thickness.
Higher film thicknesses however typically brings its own problems. Thicker films tends to mean increasing orientation, in particular with high molecular weight material. The increasing orientation in thicker films e.g. 60 μm films is characterised by low Elmendorf tear resistance in the machine direction. The combination therefore of good tear in thicker films is doubly challenging.
A further problem with MDO films is that in order to achieve a final film thickness of, for example, 60 to 80 μm, it is necessary to produce a primary film in the thickness of 360-480 μm. This results from the fact that the film is often stretched around 6 times in order to achieve the right balance of film stiffness and toughness. As the suitable process technology for the used materials is blown film, it is typically not possible to achieve such thicknesses with primary films. In order to achieve thicker films therefore, the blown film is “blocked” at the nip rolls. Typically therefore, where a film having an ABC type structure is used, layer C is the blocking layer. As the three layer coextruded film exits the die in the form of a bubble, the bubble is optionally cut and then two halves forced together to effectively form a ABCCBA type structure. In this way, the film thickness is effectively doubled and the desired initial film thickness achieved. This is called film blocking in the art.
The materials used for the blocking layer are very soft polyethylene copolymers such an ethyl acrylates with low Vicat softening point and high stickiness. These could have negative impact on the final film properties e.g. when the blocking is not properly done (leading to air inclusion). In addition these polymer materials are rather expensive and therefore a significant cost contributor to the final film recipe. Another disadvantage is that only symmetric film constructions can be produced which is a disadvantage when it comes to sealing properties.
Surprisingly, the present inventors have found that when using a particular multimodal LLDPE in MDO films, it is possible to achieve a thick primary film and thus avoid the use of a blocking layer completely. With the right balance of high and low molecular weight components, and an appropriate density, a thick primary film with thickness of, for example, 240 μm or more can be produced, which still provides, after stretching, high mechanical strength, in terms of dart drop and stiffness. The polymer also possesses excellent Elmendorf tear in the machine direction.
This also allows the manufacturer to produce asymmetric films which give e.g. a broader sealing range together with high mechanical strength.
The use of LLDPE materials in MDO films is not new. In EP-A-1941998 an MDO film is described in which the outer layer is formed from a blend of Ziegler Natta multimodal LLDPE and a metallocene produced LLDPE. The films described are multilayer films.
In EP-A-1941999, the outer layer of the MDO films described also contains a blend of in an MDO film comprises a blend of Ziegler Natta multimodal LLDPE and a metallocene produced LLDPE. The films are again multilayer and comprise a (C) layer formed from an acrylate.
Moreover, the multimodal LLDPE of use in this invention, based on a homopolymer component and a terpolymer component is not itself new. It is generically suggested in references such as those above and described in EP-A-1333044 or EP-A-1472298 for use in injection moulding. The use however of the multimodal LLDPE defined herein in the specific MDO films defined herein is new.